Commercially available micro-CT (computer tomography) systems have failed to meet the challenging demands for small animal imaging, beyond the single requirement for good spatial resolution. Specifically, they have failed to take into account that the thousand times smaller volume in mice, compared to humans, is also coupled with 10 times faster heart rates, and increased respiratory rates and circulation times. Thus, the currently available systems have been limited to either: (1) in-vivo volume CT, which is primarily useful for co-registration of anatomical with functional images provided by positron emission tomography (PET) or single photon emission computer tomography (SPECT), but is very slow (several minutes); or (2) ex-vivo high spatial resolution CT, with good contrast resolution for specimen imaging but at too high dose for live subjects. Thus, these commercial systems all fail to address significant applications for micro-CT, such as tumor perfusion and cardiac ejection fraction studies. The goal of this project is to develop a next generation micro-CT for pre-clinical imaging for drug development and medical research. The system will be designed to provide clinical quality CT images in vivo in mice and rats, with performance specifications optimized to accommodate the demanding requirements of the most critical areas currently in medical research. Specifically, the instrument will provide very high spatial resolution (< 100 mm), ultra fast frame speeds (< 1 second), and low radiation dose to the subject. This will be achieved utilizing a photon counting mode for x-ray acquisition, instead of the standard current integration mode. The system will be capable of gross energy information and energy binning, and thus will provide "Color-CT(tm)" capability, allowing tissue compositional analysis. To accomplish these advances, the system will feature advanced CdZnTe solid state detectors, coupled to high speed integrated circuits for signal processing, to achieve performance characteristics not currently possible from conventional technologies used by the major CT manufacturers. During the Phase I project, we will develop an x-ray detector module incorporating a 4 x 64 pixel CdZnTe detector, coupled to high speed custom integrated circuit readout electronics. The detector performance will be evaluated in response to a high intensity x-ray flux, on an existing CT gantry, with regard to spectral quality, energy resolution and count rate performance. The detector capabilities will be evaluated in a side-by-side comparison with a commercial integrating mode volume-CT detector, with regard to required system specifications such as contrast, dose, and acquisition speed. In Phase II, a fast dynamic multi-slice Color-CT(tm) scanner prototype will be developed, employing multiple detector modules in a novel system design incorporating a very fast x-ray source.